1. Field of the Invention
This invention relates generally to non-destructive surface measurement. More particularly, it relates to the measurement of surface elevation, surface slope, and/or optical power of the human cornea.
2. Background
The cornea, the front surface of the eye, provides about two-thirds of the eye""s refractive power and therefore is important to quality of vision. Accurate measurement of the shape of the human cornea is of great concern in the field of ophthalmology and optometry. The accuracy of these measurements directly affects the ability to detect early cornea disease, accurately fit hard contact lenses, and compute the correct power for a phakic or aphakic intraocular lens. Most importantly, accurate measurement of a cornea, including minor surface imperfections, is critical to perform successful custom corneal refractive laser surgery to correct myopia or hyperopia.
One method of measuring the shape of the anterior cornea, which is widely used today, is a reflected target corneal topography system, e.g., as shown in FIG. 10. Using this reflected target system, corneal topography is typically measured using a series of concentric lighted rings, known as a keratoscope pattern.
FIGS. 10 and 10A show a conventional monocular corneal topography system.
In particular, as shown in FIG. 10, an illumination source (not shown) projects infrared rays through a keratoscope target 1010, which comprises, e.g., illuminated concentric rings, as shown in FIG. 10A. The rays are projected onto the cornea 1012 of a patient""s eye 1014. The cornea in part reflects the rays. A front view camera with lens 1006 captures the rays and focuses them onto a CCD 1004. The rays are in the form of a keratoscope pattern 1008, e.g., a reflected image of rings. A computer 1002 processes the image to detect the rings, to apply a reconstruction algorithm to extract elevation and slope or curvature data, and to generate and display a color-coded contour map for interpretation by a health care professional.
Typical methods used by these instruments make some assumptions about the shape of the cornea to reconstruct and extract the desired data. Assumptions are required due to the non-unique nature of the acquired image.
For example, FIG. 11 depicts an exemplary problem of computing surface points using a conventional monocular reflective corneal topography technique and apparatus.
In particular, as shown in FIG. 11, a given target point T1 is reflected off the cornea 1012 and captured by a lens L1. In the digital reconstruction of the image, there are, e.g., three possible xe2x80x9csurfacexe2x80x9d points, S1, S2 and S3, with different elevations and surface normals, that could have reflected target point T1. In this example, only surface point S2 is correct because it is the only point actually located on the surface of the cornea. Therefore, a method is required to select a xe2x80x9cbestxe2x80x9d surface point.
A conventional assumption is that the curvature between data points is constant. This was the assumption made by several researchers and manufacturers including Wang (Wang 1998), Klein (1992), Campbell (1997), van Saarloos (1991), Mattioli (1997), and Brenner (1997).
However, the assumption of constant curvature is not entirely satisfactory because it can lead to errors that accumulate, as pointed out by the same researchers. Some attempts to overcome these limitations have had some success (e.g., Halstead et al., (1995) and Klein (1992)), but have not been shown to be clinically viable.
One approach by Sarver and Broadus (U.S. Pat. No. 6,079,831) combines both scanning slit and reflection target techniques. This approach represents an improvement on the accuracy of previous corneal topography instruments, but the resulting instrument is complex and expensive to produce.
There is a need for an improved reflective target corneal topography technique and apparatus that is simple to produce and that provides superior measurement results.
An improved reflective target corneal topography system includes a stereo image reconstructor that generates surface elevation data, surface normal data, and surface power data without the traditional need for assumptions about the shape of the surface being tested. The reflective target corneal topography system further includes stereo optics, with image detection devices in each optical device, to preferably allow substantially simultaneous stereo image acquisitions.